Electroformed article



April 2, 1946. E. M. wlsE ETAL ELECTROFORMED ARTICLE Original Filed Feb.26, 1940 .fand/V0 M M55, vp PHY/wvo E 14A/55 0. a @b5 Oe F8 7 Cw m z Z 2@weeg Fw" Patented Apr. 2,1946- Q p ELEcrnoFonMEn ARTICLE EdmundMerriman Wise, Westfield, and Raymond Francis Vines, Plainfield, N. J.,assignors to The International Nickel Company, Inc., New York, N. Y., acorporation of Delaware I Original application February 26, 1940, SerialNo. 320,850. Divided and this application November 30, 1942, Serial No.467,320

5 Claims.

The present invention relates to electroformed articles and, moreparticularly, to a process for electroforming articles including mirrorsand searchlight reflectors, especially reflectors having highreflectivity and resistant to severe corrosive conditions.

The prior art electroforming processes for producing mirrors andsearchlight reflectors have certain inherent disadvantages which hinderor prevent the deposition of a sufficiently thick, corrosion resistingprecious metal electroplate to insure freedom from porosity and thus toobtain the required degree of protection. For example, if a suillcientthickness of platinum metal plate is deposited to obtain adequatecorrosion resists ance, the'refiecting surface is rough and dull andmust be polished to raise the reflectivity to the required minimum. Thisis a costly operation, removes some precious metal and destroys thecontour of the refiecting vsurface and in some cases, as in reproducinggratings, is iinpossible. Another disadvantage of the prior artprocesses is the inability inherent in them to produce satisfactoryrhodium surfaced palladium plated reflectors owning to the fact thatwhen rhodium is depositedl upon palladium the codeposited hydrogencauses puckering and roughening of the underlying palladium plate, thusimpairing the smoothness of the reflecting surface.

Furthermore, where a highly polished base metal or silver reflectingsurface is to be pro tected by e1ectro-depositing a non-tarnishing noblemetal upon it, difficulty is encountered in securing adherence to thevery smooth surface and the plating baths must he restricted to those ofthe low metal ion type in an effort to reduce local galvanic effectswhich would roughen the base metal and impair the adherence andcontinuity of the noble metal deposit.

In prior efforts to secure better corrosion resistance various metalssuch as gold, palladium and the like have been deposited upon the basemetal and then the final reecting surface, generally rhodium, wasapplied by Aelectrodeposition but even such composite coatings haveproved inadequate for marine use particularly in submarines, so recoursehas been made to reflectors of highly corrosion resistant high chromiumcon- N tent alloys which are not only difficult to forge, grind andpolish and hence are very expensive, but possess inferior reflectivity.Glass -reflectors are unsuitable for this application because of theirfragility and vulnerability to even small projectiles. l

From the foregoing it will be readily appreciated that the art hasfailed t provide a process whereby mirrors may be electro-formed to havea precious metal surface of such thickness as to provide the corrosionresistance necessary to meet the rigorous requirements of marine use 5vand at the same time provide the eflicient reflecting surface necessaryfor such service. We have discovered that reflecting surfaces may beelectro-formed to have a precious metal or platil num metal surfacewhich meets the most rigorous requirements.

It is an object of the present invention to provide electroforrnedarticles having a platinum metal surface.

It is another object of the present invention to provide electroformedarticles having a precious metal reflecting surface capable ofwithstanding normal corrosive conditions for longer periods than priorart reflectors without appreciable reduction of the reflectivitythereof.

It is Sa further object, of the present invention to provideelectroformed articles having a precious metal reflecting surface havinga higher reiiectivity in the unpolished state than prior electroformedreflecting surfaces.

The present invention also contemplates the provision. of electroformedarticles having preclous metal or platinum metal reflecting surfaceshaving high corrosion resistance sumcient to meet the requirements ofmarine use and having with alloys of high chromium content.

Other objects and advantages will become apparent from the followingdescription taken in conjunction with the .drawing in which Figs. 1-10/areillustrative in a schematic manner of the steps in a processemploying the principles of the present invention.

Broadly speaking, electrodeposits tend to become progressively rougherwith increase in 4@ thickness. Difficulties also arise in the prior artprocesses due to galvanic effects occurring when a more noble metal isdeposited onl a less noble one. It is also to be noted that theadherence of successive layers of electrodeposited metals improves withincreasing roughness of the surface to which the deposit is applied.

In the present invention all of the above factors are in optimumrelation because the most noble metal of the final structure, isnormally de- 09 posited rst and upon the smoothest surface;

while the subsequent plates of lower nobility are deposited uponprogressively rougher surfaces.

By this procedure a wide variety of plating baths are suitable and thethickness of the highly corrosion resistant layer or layers is limitedonly by cost and not by loss of smoothness. peeling or otherlimitations. On the other hand, in the prior art a refiectivitycoeicient higher than is .possible process the major corrosion resistingplate was applied to a very smooth, less noble surface and the finalreflecting surface tended to become progressively rougher with theincrease in the thickness of the protective plate and in some cases,particularly where palladium Was used for the protective plate, thesurface would suffer further roughening when the final reflectiverhodium plate was applied, this aggravated roughness being due tohydrogen absorbed by the Palladium during rhodium plating.

From the foregoing it will be recognized that a reflector produced inaccordance with the principles of the present invention can readily bedistinguished from prior art electroformed precious metal reflectors.When a prior art electroformed reflector having an unbuffed preciousmetal reecting surface and corrosion resistant layer is examinedmicroscopically in section, it will be found 'that the unbuiedreflecting surface of the precious metal layer is rougher than theinterface betweenthepreciousmetalreecting-surface-andcorrosion-resistant layer and the backing layerof non-noble metal. In distinctcontrast, the unbuied reflecting surfaceof the precious metal reflecting-surface-and-corrosion-resistant layeris smoother than the interface between the precious metal layer and thebacking layer of non-noble metal of a reiiector produced in accordancewith the principles of the present invention.

In the prior art processes, a glass or other matrix having a contour thereverse of that desired in the nal product was employed. In contrast toprevious practice the preferred embodiment of our invention employs aglass or metal matrix possessing the contour desired in the finalproduct. This matrix of the desired contour, if of glass, is carefullycleaned. and coated with a conducting layer such as silver deposited bythe Brashear or other wet process, by vacuum sputtering or distillation,or by other convenient means and is then plated with copper from astandard copper sulphate bath (it may be remarked that cyanide bathsappear less suitable as they lare apt to damage the silver surface) to athickness suflicient toavoid deformation when thev electro-formed shapeis removed from the matrix. After stripping the electroformed shape fromthe matrix the copper surface is protected with a stop-off and thesurface which had been in contact with Vthe glass matrix is plated withAa thin layer of rhodium followed by a thicker layer of the 'majorcorrosion resisting metal, preferably platinum, palladium deposited fromthe complex nitrite or similar' baths or from .newly devised high metalion high speed plating baths or gold depositedfrom a double cyanidebath. The corrosion'resistant surface is then vbacked up by base metaldeposits, generallyv copper and nickel in one or more layers, to providethe strength reoperation. Final plating with rhodium cannot be donewhere palladium is used for the principal corrosion resisting surface asthe hydrogen deposited along with the rhodium will cause slightpuckering of the palladium thus impairing the micro flatness of thesurface. Y

Metal matrices can also be used in which case the deposition of theinitial conducting layer of silver or other metal may be dispensed withbut generally the surface of the matrix will require surface treatmentby one of the processes known to the art to prevent an excessively goodbond developing on electroplating with copper or nickel. In the absenceof such special treatments 'it is diicult or impossible to remove thepreliminary electroformed shape without damaging it or the matrix.Further steps in the process are the same as described for a glassmatrix.

It has not been proved feasible to produce mirrors by depositing rhodiumdirectly upon a metal matrix and then to deposit palladium or platinumfollowed by nickel and copper due to the diiilculty of removing thecomposite from the metal matrix due to the excessively good bonddeveloped between the rhodium and the metal matrix. However, thisdifliculty can be avoided by applying a thin nickel plate, preferablybright nickel to the matrix, the latter having been treated to preventthe formation of an excessively good bond. This thin nickel plate isthen coated with rhodium, platinum, palladium or gold followed by aheavy nickel and copper plate. The composite is then stripped from themetal matrix and the thin nickel plate is then stripped olfelectrolytically or chemically as described.

In order that those skilled in the art may have a better understandingof the principles of the present invention for the preparation ofelectroformed articles having high 4reflectivity without polishing, thepreparation of a reflector which has given satisfactory results bothfrom the standpoint of corrosion resistance and the standpoint ofreflectivity will be described in conjunction with Figs. 1 to 10. l

A matrix 2| (Fig. 1) having the contour desired in the' finishedreiector is provided with a silver coating 22 on the reecting or concavesurface (Fig. 2). A copper layer 23 is then deposited yupon the silvercoating to provide an article with sufficient strength for handling insubsequent treatments (Fig. 3).

concavel surface of the copper is then protected quired in` the nalproduct. The stop-off is then' removed from the preliminary electroformand an acid resistant stop-off is appliedv to the last dev posited basemetal surface. .The preliminary elec-v tro-form is then removed bytreatment with nitric acid or by electrolyzing it as anode in dilutesulphuric acid. Whenthe stripping has been completed the rhodiumreiiecting surface will be exposed and will possess exactly the contourof the original matrix. A

If platinum is used for the principal corrosion resisting layer andwhere its slightly lower reectivity is suitable, the preliminary rhodiumplate may be dispensed with or the thin rhodium plate may be appliedtothe platinum as a nal `5 and 6).

with lacquer, wax or other stop-01T 24, and the composite removed fromthe-matrix (Fig. 4). Rhodium 25 which provides the ultimate reflectingsurface is then deposited upon the convex silver surface 22 and backedup with a corrosion resistant layer of palladium or platinum 26 (Figs.Nickel or copper or both 2l are then deposited upon the corrosionresistant layer 26 tof provide the strength vandrigidity required in thefinished product (Fig.v 7.). After protecting the last depositedstrengthening layer' or layers 21 with wax, lacquer or other stop-o3 28-(Fig. 8)

theV stop-off layer24 is removed (Fig. 9) and the initial transitorysilver and copper layers 22 and 23 then areremoved with nitric acid orby electrolysis exposing the rhodium reflecting surface and thuscompleting the article (Fig. i0).

Plane-mirrors can be made in a manner similar to the foregoing. Forexample, optically fiat glass is silvered, copper plated on'` thesilvered surface and the two layers are then removed from the glass.After fstopping off the copper surface, rhodium, platinum or palladiumor coml The exposed v binations thereof are deposited on the silversurface and the platinum metal layer or layers are stiii'ened by abacking of heavy copper or nickel plate. The initial stop-off layer isremoved and the second heavy copper or nickel layer protected by astop-off layer. The initial transitory 1ayers of silver and copper areremoved electrolytically in sulfuric acid after protecting the copper ornickel backing layer with a stop-olf, which is a material resistant tothe agent or agents employed to remove the initial silver and copperlayers.

The preparation of the mold for the deposition of the initialsilverlayer may be carried out in any suitable manner but We prefer topolish a glass surface with suspension of chalk or other suitable agentsand cotton, applying plenty of pressure to the cotton. Care must beexercised not to contaminate the glass surface by contact with greasyobjects such as the hands of the operator. By the use of rubber glovesthis difficulty is obviated. After polishing the glass mold is rinsedwith water, immersed in a cleaning solution, such as 50% by volumenitric acid, and swabbed. It is rinsed again with Water without removaltherefrom. At this point we have found that, in accordance with acceptedmethods of preparing glass surfaces for the deposition of silvermirrors, it is advisable to swab the polished glass surface with astannous chloride solution of say about 15% concentration. The polishedglass is again rinsed with Water without removal therefrom and finallyrinsed with distilled water and allowed to stand just covered with wateruntil ready for silvering. We have found that the inclusion of the stepof treating the polished glass surface with stannous chloride solutionimproves the adherence of the silver film applied later.

A silver film is applied to the polished and cleaned glass surface inthe usual manner employing a suitable silver solution such as that ofBrashear or variations thereof. The silvered glass mold is then readyfor the deposition of successive films as described hereinbefore.

We have found it advisable when depositing copper upon the silver filmat room temperature to employ a high starting current of the order ofabout 50 amperes per square foot. We have also encountered Adifficultieswhen attempting to deposit palladium, platinum, rhodium or nickel orcopper (from a cyanide copper solution) upon the silver lm because therehas been a tendency for the silver film to ake.

We have' found that reflector surfaces comprising a film of platinumabout x10-4 inches thick gives satisfactory service while a' reflectorsurface plated with 5X 10-4 inches of platinum and then nished with anelectrodeposit of.

rhodium 2X 10-6 inches thick gives even higher reflectivity and verysatisfactory service. However, we prefer to employ palladium about5x10*4 inches to about 2X 10-3 inches thick as the corrosion resistinglayer which is surfaced with a film of rhodium about 2 106 inches toabout 5 1O5 inches thick to provide the maximum reflectivity coupledwith minimum cost.

Electroformed reflectors produced by means of the present process andhaving a palladium layer about 5X 10-4 inches thick, the exposed surfaceof which provides the reflecting surface of the reflectors, have beensubjected to a severe salt spray test under conditions which haveresulted in the complete failure of prior art electroformed preciousmetal reflectors. The electroformed reflectors produced in accordancewith the principles of the present invention under test have withstoodthe rigors of this severe salt spray test with unimpaired reflectivityand without corrosion. Similar electroformed articles but havingaplatinum layer about 5 10-4 inchesy thick in place of the palladiumlayer have likewise been tested under the same conditions and have alsobeen found satisfactory, whereas prior art electroformed precious metalreflectors have failed completely.

Furthermore, an electroformed reector produced in accordance with priorart methods was subjected to an atmospheric corrosion test together witha reflector produced by the process of l v' the present invention. Thesite of the test was a shore front about 250 yards from the beach withthe prevailing winds from the south and southwest. The test reflectorsfaced south at an angle of 30 from the horizontal. The testreilector'prepared in accordance with prior art electroforming methodsand designed as A hereinafter consisted of a., reflecting surface layerof rhodium about 1 10-5 inches thick, a nickel undercoat about 2 103inches thick and the remainder copper. The simultaneously testedreflector electroformed in accordance with the principles of the presentinvention and designated as B hereinafter consisted of a reflectingsurface of rhodium 1 10*5 inches thick, a palladium undercoat 2 104inches thick and the remainder copper. At the end of about four monthsexposure to sea air carrying a considerable amount of spray, testspecimen A showed a myriad of tiny pits in many of which a greenishcorrosion product could be detected, On the other hand, the surface oftest specimen B was entirely free from pits. The'foregoing comparativeltest of a prior art electroformed precious metal reflector and areflector made in accordance with the principles of the presentinvention clearly proves the complete superiority of the presentelectroformed precious metal reflector. The foregoing test also clearlyproves that, whereas prior art electroformed precious metal reflectorsare unacceptable .for use under severe marine conditions, the presentelectroformed refiectors are acceptable.

In producing reiiectors in accordance with the principles of the presentinvention the following baths have been used under the conditions setforth to plate nickel, copper, platinum, palladium and rhodium.

Copper plating (for protective layer and for Agitated C. D, at roomtemp. 5 0 amp/ft2 Nickel plating Grams per liter Nisoi 33o Nich 16.5H3BO3 30 4 c. c. ofi-i202 (3%) per liter Agitated C D. at 130 C. 50amp/ft.2 pH (preferred) 2.2.

Palladium plating (employing porous cell) Add NHlOI-I and boil for sometime; evaporation made up with 5% (by vol.) ammonia solution. AgitatedTemp. 90 C., C. D. 'l amps/ft.

Rhodium plating Bakers solution: c. c./1iter N0. 219 20 H2SO4 (conc.) 20

Temp. 40 C., C. D. 5 and 40 amps/ft.2

The present application is a division of our copending U. S. applicationSerial No. 320,850 led February 26, 1940, now Patent No. 2,305,050issued December 15, 1942.

Although the present invention has been described in conjunction withcertain preferred embodiments thereof, it is to be understood thatvariations and modifications thereof can be made as those skilled in theart will readily understand. Thus the expression precious metal orplatinum metal includes gold, platinum, palladium, rhodium and iridium.Similarly, for copper and nickel may be substituted cobalt, or lessdesirably, iron and iron-nickel alloys. Such variations andmodifications are -to be considered within the purview of thespecication and the scope of the claims.

Furthermore, those skilled in the art know of the diflcultiesencountered particularly in making dat reectors. A method by which thesedifficulties can be overcome involves the use of a preliminaryelectroform of such thinness that it does not have the structuralstrength requisite for use as an electroform per se. This modificationof the present invention involves preparing a matrix having the desiredoptical properties. The matrix may be of any'suitable material,preferably of glass having an optically at surface of the matrix. Thechoice of the metal of this very thin film is dependent upon theadherence of the metal of the very thin film to the mirror surface ofthe matrix. The adherence should be suiicient that the thin lm doesnotfracture or curlor blister but still insufficient to markedly increasethe diiculties of separating the thin fllm from the mirror surface ofthe matrix after the previous metal reflector h'as been produced. Gold,nickel and similar metals may be employed as the metal of the thin film.Gold may be sputtered upon the mirror surface of the matrix or nickelmay be deposited from nickel carbonyl. Each method of producing the verythin film has given satisfactory results. After the mirror surface ofthe matrix has been covered with the very thin lm of metal, a very thinbut heavier layer of non-noble metal, for example, copper iselectrodeposited upon the exposed surface of the very thin nlm firstdeposited. Thereafter the precious metal layer or layers of the reectoris built-up upon the exposed surface ofthe electrodeposited non-noblemetal in the manner set forth hereinbefore and a supporting layer ofnon-noble metal electro-deposited upon the exposed surface of theprecious metal layer as described hereinbefore.

When the electro-deposition of the non-noble metal structural layer hasbeen completed, the precious metal reflector with its structural layerof non-noble metal and the very thin preliminary electroform areseparated from the matrix in any suitable manner, for example, byheating the composite article sufficiently to cause a separation betweenthe rst layer of metal deposited upon the mirror surface of the matrixand the matrix. However, the temperature to which the composite articleis raised should not be high enough to cause any appreciable warping ofthe reflector surface of the precious metal refiector. After theprecious metal reflector and the preliminary electroform have beenseparated from the matrix, the exposed surface of the non-noblestructural layer of the precious metal reflector is protected by astop-off layer`of lacquer or wax or the like and the preliminaryelectroform dissolved chemically or electrochemically to expose theprecious metal reflector surface of the electroformed reflector. In theunbuffed state, the reiiector surface thus obtained is far smoother thanthe interface between the metal of the refiector surface and theprecious metal of the corrosion resistant layer or the interface betweenthe precious metal and the non-noble metal structural layer.

We claim:

1. As a new article of manufacture, a parabolic multi-layer compositemetal reiiector comprising a transitory layer of copper-silver laminaein interfacial contact formed upon and lifted from a glass matrix, saidtransitory layer having a concave copper outer surface and a convexsilver inner surface, an electrodeposited rhodium layer about 2 10-6inches to about 5 X 10-5 inches thick onsaid convex silver surface, acorrosionresistant layer of electrodeposited palladium about 5 X 10-4inches to about 2 X l03 inches thick on the convex surface of saidrhodium layer, and at least one supporting layer of electrodepositednon-noble metal selected from the group consisting of copper, cobalt,nickel, iron and ironnickel alloys in sufficient thickness to protectthe aforesaid precious metal layer from deformation on the convexsurface of said palladium layer, the said transitory layer serving'as amatrix and being adapted for separation from the remaining layerswhereby the concave rhodium surface is vexposed as a reflecting surface.

2. As a new article of manufacture, a parabolic multi-layer compositemetal reflector comprising a transitory layer of copper-silver laminaein interfacial contact formed upon and lifted from a matrix, saidtransitory layer having a concave copper outer surface and a convexsilver inner surface, a layer of electrodeposited rhodium on said convexsilver surface, an electrodeposited palladium layer considerably thickerthan said rhodium layer on the convex surface thereof, and a layer ofelectrodeposited non-noble metal of structural strength supporting saidprecious metal layers on the convex surface of said palladium layer,said transitory layer serving as a matrix and being adapted forseparation from the remaining layers whereby .the concave rhodiumsurface is exposed as a reflecting surface.

3. As a new article of manufacture, a multilayer metal compositereflector comprising a 4transitory layer of copper-silver laminae ininterfacialcontact, said transitory layer having a concave copper outersurface and a convex silver inner surface, an electro-deposited rhodiumlayer on said convex surface, an electrodeposited layer of a noble metalselected from the group consisting of platinum and palladium as theconvex surface of said rhodium layer, and an electrodeposited layer of'a non-noble metal selected from the group consisting of nickel andcopper on the convex surface of said noble-metal-group metal, the saidtransitory layer serving as a matrix and being adapted for separationfrom the remaining layers whereby the rhodium layer is exposed as anoptically correct reflecting surface.

4. As a new article of manufacture, a multilayer metal compositereflector comprising a transitory layer of copper-silver laminae ininterfacial contact, said transitory layer having a concave copper outersurface and a convex silver inner surface, an electrodeposited rhodiumlayer on said convex silver surface, an electro-deposited layer of aplatinum metal-other-than-rhodium on the opposite face of said silverlamina, an electrodeposited layer of a platinum metal-otherthan-rhodiumon the opposite face of said rhodium'layer, and an electrodepositedlayer of non-noble metal on the opposite face of said piatinummetal-other-than-rhodium layer, the said transitory layer serving as asilver-surfaced ma- 20 trix and being adapted for separation from the

